1. Field of the Invention
The present invention relates to a method and apparatus for recovering energy from wastes, and more particularly to a method and apparatus for recovering energy by gasifying wastes at a relatively low temperature and then at a relatively high temperature, and thereafter either recovering thermal energy from the generated gas to generate electric energy or combusting the generated gas to drive a gas turbine to generate electric energy and also recovering waste heat from the gas turbine to generate electric energy.
2. Description of the Prior Art
It has been customary to treat various wastes such as municipal wastes, plastic wastes, sewage sludge, industrial wastes by incineration to reduce their volume or to discard them in an untreated state to landfill sites.
In recent years, the recognition of limited availability of fossil fuels and demands for converting wastes into useful resources have increased trends toward effective utilization of energy of wastes.
Dumping the wastes in an untreated state in landfill sites has become more difficult because of scarcity of the landfill sites, and has not been allowable from the viewpoint of environmental conservation.
Therefore, an energy recovery system has lately been put to practical use, the energy recovery system having an incinerator for combusting wastes, a waste heat boiler combined with the incinerator for recovering heat from high-temperature exhaust gas ascending in the incinerator to generate steam, and a turbine that can be driven by the steam for rotating an electric generator coupled therewith to generate electric energy.
Further, there has been developed another similar energy recovery system in which heat is recovered from the waste heat boiler combined with the incinerator to generate steam, and the temperature of the generated steam is further increased by waste heat from a gas turbine for allowing the electric generator to generate electric energy with higher efficiency.
However, the above-mentioned energy recovery systems which employ an incinerator for combusting wastes suffer limitations on efforts to increase the temperature of exhaust gas and hence steam. Another problem of the conventional energy recovery systems is that generation of dioxins is unavoidable because it is impossible to decompose dioxins that are produced by the incinerator and the rate of resynthesis of dioxin at the boiler is high.
In addition, since solids such as ash generated by the incinerator contain heavy metals, those heavy metals tend to be separated out in landfill sites where the solids are finally disposed of.
In order to prevent dioxins from being generated and also heavy metals from being separated out, efforts are being made to develop a system for gasifying and melting wastes rather than incinerating wastes. The gasifying and melting system gasifies wastes at a relatively low temperature and then combusts gaseous material and char at a relatively high temperature. Since dioxins are decomposed at the high temperature, the system does not discharge dioxins. Furthermore, no heavy metals will be separated out in landfill sites because ash content is melted into slag.
In the gasifying and melting system, it is necessary to provide a process for cooling the generated gas when the gas is to be purified. In this cooling process, therefore, the thermal energy which the generated gas has is lost, and hence cannot effectively be utilized. Furthermore, no attempt has been made to use the generated gas directly as a fuel for generating electric energy.